Electrode structure and method for fabricating the same

ABSTRACT

The electrode structure of the invention includes a p-type Al x  Ga y  In 1-x-y  N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer and an electrode layer formed on the semiconductor layer. In the electrode structure, the electrode layer contains a mixture of a metal nitride and a metal hydride.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode structure for a p-typeAl_(x) Ga_(y) In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor device,one of the Group III-V compound semiconductor devices containingnitrides, and a method for fabricating the same. More specifically, thepresent invention relates to an electrode structure having an idealohmic contact showing an extremely small contact resistance between asemiconductor layer and an electrode layer and a method for fabricatingthe same.

2. Description of the Related Art

Generally, in fabricating an electrode structure for an Al_(x) Ga_(y)In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor device, nitrogen, oneof the elements constituting the semiconductor, is likely to dissociatefrom the surface of a semiconductor layer in the electrode structurewhen the semiconductor layer is formed. Therefore, it is difficult toproduce crystals satisfying a desirable stoichiometric ratio. When thedissociation of nitrogen forms the vacancies inside the crystalstructure of the semiconductor layer, the conductivity type of thesemiconductor layer turns into n-type. Therefore, in fabricating anelectrode structure for an Al_(x) Ga_(y) In_(1-x-y) N (0≦x≦1, 0≦y≦1,x+y≦1) semiconductor device, it is difficult to form a p-typesemiconductor layer.

As a method for turning a semiconductor layer containing nitrogen into ap-type semiconductor layer, it is well known to dope the semiconductorlayer with magnesium (Mg) as an acceptor impurity. However, a p-typeAl_(x) Ga_(y) In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layerformed by a commonly-used metalorganic chemical vapor deposition (MOCVD)method contains a large amount of hydrogen inside semiconductorcrystals. A part of the hydrogen atoms are bonded with the Mg atomsfunctioning as an acceptor impurity, thereby preventing the Mg atomsfrom functioning as an effective acceptor. In order to activate the Mgatoms as an acceptor impurity, the semiconductor layer is subjected toan electron beam irradiation process or an annealing process within anitrogen environment, thereby forming the p-type Al_(x) Ga_(y)In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer. However, thep-type semiconductor layer subjected to such a process does not have acarrier density high enough to form an ideal ohmic contact between asemiconductor layer and an electrode layer.

On the other hand, various electrode structures usable for Group III-Vcompound semiconductor devices containing nitrides such asblue-light-emitting diodes have been conventionally developed. In theproposed electrode structures, various kinds of metals are used to formthe electrode layer. For example, in order to form the electrode layerfor a p-type Al_(x) Ga_(y) In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1)semiconductor device, gold (Au) is most commonly employed("P-GaN/N-InGaN/N-GaN Double-Heterostructure Blue-Light-EmittingDiodes", S. Nakamura et al., Jpn. J. Appl. Phys. (1993) p. L8). JapaneseLaid-Open Patent Publication No. 5-291621 discloses that nickel (Ni),platinum (Pt) and silver (Ag) may be used in place of Au as the metalsfor forming the electrode layer.

However, in the case of using Au for the electrode layer, the contactresistance between the electrode layer and the semiconductor layer islarge, and therefore an ideal ohmic contact cannot be obtained. Inaddition, the adhesiveness between the electrode layer and thesemiconductor layer is inferior and the physical strength of thesemiconductor device becomes disadvantageously weak.

On the other hand, in the case of using Ni, Pt or Ag for the electrodelayer, the resulting adhesiveness is surely superior to that of Au, sothat a more ideal ohmic contact can be obtained as compared with thecase of using Au. However, in the case of using these metals, thefollowing problems are caused, for example. In a light-emitting diodeusing these metals for the electrode layer, a differential resistancevalue at the current value of 10 mA is large, i.e., several tens of Ωs.In other words, such a light-emitting diode has a high operationalvoltage, judging from the current-voltage characteristics thereof. Inaddition, since a laser diode using these metals for the electrode layerhas a small electrode area, the contact resistance is increased ascompared with a light-emitting diode. As a result, the operationalvoltage of the laser diode becomes larger than that of a light-emittingdiode. This is why an electrode structure having a sufficiently idealohmic contact cannot be obtained even by the use of these metals.

In consideration of these problems, a conventional electrode structureusing these metals for the electrode layer cannot be regarded as anideal electrode structure for a p-type semiconductor device. Therefore,an electrode structure for a p-type semiconductor device having an idealohmic contact showing an extremely small contact resistance between asemiconductor layer and an electrode layer is eagerly sought.

SUMMARY OF THE INVENTION

The electrode structure of the invention includes a p-type Al_(x) Ga_(y)In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer and an electrodelayer formed on the semiconductor layer. In the electrode structure, theelectrode layer contains a mixture of a metal nitride and a metalhydride.

In one embodiment, the metal nitride is selected from a group consistingof ScN, TiN, VN, CrN, ZrN, NbN, LaN and TaN.

In another embodiment, the metal hydride is selected from a groupconsisting of YH₂, CeH₂, PrH₂, NdH₂, SmH₂, EuH₂, YbH₂, HfH₂, PdH, TmH,ErH, HoH, DyH, TbH and GdH.

According to another aspect of the invention, a method for fabricatingan electrode structure is provided. The method includes a step offorming an electrode layer on a p-type Al_(x) Ga_(y) In_(1-x-y) N(0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer. The electrode layer is formedby sequentially depositing a nitride forming metal and a hydrogenabsorbing metal on the semiconductor layer.

In one embodiment, the method for fabricating an electrode structurefurther includes a step of heat-treating the semiconductor layer and theelectrode layer after the nitride forming metal and the hydrogenabsorbing metal are sequentially deposited on the semiconductor layer.

According to still another aspect of the invention, a method forfabricating an electrode structure is provided. The method includes astep of forming an electrode layer on a p-type Al_(x) Ga_(y) In_(1-x-y)N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer. The electrode layer isformed by simultaneously depositing a nitride forming metal and ahydrogen absorbing metal on the semiconductor layer.

In one embodiment, the method for fabricating an electrode structurefurther includes a step of heat-treating the semiconductor layer and theelectrode layer after the nitride forming metal and the hydrogenabsorbing metal are simultaneously deposited on the semiconductor layer.

According to still another aspect of the invention, a method forfabricating an electrode structure is provided. The method includes astep of forming an electrode layer on a p-type Al_(x) Ga_(y) In_(1-x-y)N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer. The electrode layer isformed by depositing an intermetallic compound containing a nitrideforming metal and a hydrogen absorbing metal on the semiconductor layer.

In one embodiment, the method for fabricating an electrode structurefurther includes a step of heat-treating the semiconductor layer and theelectrode layer after the intermetallic compound is deposited on thesemiconductor layer.

Thus, the invention described herein makes possible the advantages of(1) providing an electrode structure having an ideal ohmic contactshowing an extremely small contact resistance between a semiconductorlayer and an electrode layer; (2) providing an electrode structureincluding a semiconductor layer having a carrier density high enough torealize such an ideal ohmic contact; (3) providing an electrodestructure in which crystals having an appropriate stoichiometric ratioare formed at the interface between the semiconductor layer and theelectrode layer; (4) providing an electrode structure including ap-conductivity type semiconductor layer containing nitrogen atoms; (5)providing an electrode structure having excellent physical and chemicalcharacteristics; and (6) providing a method for fabricating such anelectrode structure.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an electrodestructure according to an example of the present invention.

FIG. 2 is an enlarged cross-sectional view showing an electrode layer inthe electrode structure according to an example of the presentinvention.

FIG. 3 is an enlarged cross-sectional view showing an interface portionbetween an electrode layer and a semiconductor layer in the electrodestructure according to an example of the present invention.

FIG. 4 is a graph showing current-voltage characteristics of therespective electrode structures of Example 1 and Comparative Example 1.

FIG. 5 is a graph showing the dependence of the current-voltagecharacteristics of the electrode structure of Example 1 upon theannealing temperature.

FIG. 6 is a graph showing current-voltage characteristics of theelectrode structures of Examples 3 and 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this specification, a "nitride forming metal" refers to a metal whosefree energy is reduced when the metal is nitrified. In addition, theexpression "an electrode layer contains a mixture of a metal nitride anda metal hydride" includes the following meanings: (1) the electrodelayer has a single-layered structure composed of the mixture; (2) theelectrode layer has a multi-layered structure including at least onelayer containing the metal nitride and at least one layer containing themetal hydride; (3) the electrode layer includes a portion containing themetal nitride and a portion containing the metal hydride; and (4) theelectrode layer includes a layer composed of a compound consisting ofthe metal nitride and the metal hydride.

An exemplary electrode structure according to the present invention willbe described below with reference to FIGS. 1 to 3.

As shown in FIG. 1, the electrode structure of the invention includes: abuffer layer 2; a semiconductor layer 3 and electrode layers 4 and 4' ona substrate 1 in this order.

The substrate 1 can be made of sapphire, SiC or the like. The thicknessof the substrate 1 is preferably in the range of 10 to 500 μm, and morepreferably in the range of 100 to 300 μm.

The buffer layer 2 can be made of GaN, AlN or the like. The buffer layer2 can be formed on the substrate 1 by a MOCVD method or the like. Thethickness of the buffer layer 2 is preferably in the range of 10 to 100nm, and more preferably in the range of 10 to 50 nm.

The semiconductor layer 3 can be made of a p-type Al_(x) Ga_(y)In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor. The semiconductorlayer 3 can be formed on the buffer layer 2 by a MOCVD method or thelike. The thickness of the semiconductor layer 3 is preferably in therange of 2 to 6 μm, and more preferably in the range of 3 to 5 μm. Theconductivity type of the semiconductor layer 3 can be controlled bydoping the semiconductor layer 3 with a p-type impurity such as Mg.

Next, the electrode layers 4 and 4' will be described. Herein, forsimplification, only the electrode layer 4 will be described except forthe portion where the electrode layer 4' is required.

Examples of the metal nitrides contained in the electrode layer 4include ScN, TiN, VN, CrN, ZrN, NbN, LaN and TaN, which are producedfrom nitride forming metals. Examples of the nitride forming metalsinclude Sc, Ti, V, Cr, Zr, Nb, La and Ta.

Examples of the metal hydrides contained in the electrode layer 4include ScH₂, YH₂, LaH₂, CeH₂, PrH₂, NdH₂, SmH₂, EuH₂, YbH₂, TiH₂, ZrH₂,HfH₂, VH, NbH, TaH and PdH, which are produced from hydrogen absorbingmetals. Examples of the hydrogen absorbing metals include Sc, Y, La, Ce,Pr, Nd, Sm, Eu, Yb, Ti, Zr, Hf, V, Nb, Ta, Pd, Gd, Tb, Dy, Ho, Er andTm.

In the previous paragraphs, Sc, Ti, V, Zr, Nb, La and Ta are cited asboth the nitride forming metals and the hydrogen absorbing metals.However, it is noted that these metals are effective only as the nitrideforming metals, not as the hydrogen absorbing metals. The reason is asfollows. In the case where these metals are deposited on thesemiconductor layer 3 to form the electrode layer 4, these metals firstreact with nitrogen so as to form the metal nitrides, but do not reactwith hydrogen. This is because the number of nitrogen atoms existing inthe semiconductor layer 3 is far larger than the number of hydrogenatoms bonded with the Mg atoms in the semiconductor layer 3.Accordingly, hydrogen existing in the semiconductor layer 3 is notattracted to the interface between the semiconductor layer 3 and theelectrode layer 4, so that the Mg atoms are not activated. As a result,since the carrier density in the interface between the semiconductorlayer 3 and the electrode layer 4 becomes insufficient, the formation ofthe electrode layer 4 on the semiconductor layer 3 by depositing thesemetals on the semiconductor layer 3 does not always improve the ohmiccharacteristics of the semiconductor device. Consequently, it ispreferable that Sc, Ti, V, Zr, Cr, Nb, La and Ta are used as thenitrogen forming metals and Y, Ce, Pr, Nd, Sm, Eu, Yb, Hf Pd, Gd, Tb,Dy, Ho, Er, and Tm are used as the hydrogen absorbing metals.

The thickness of the electrode layer 4 is preferably in the range of 100to 500 nm, and more preferably in the range of 100 to 200 nm. Thecross-sectional shape of the electrode layer 4 may be an arbitraryshape, e.g., circular, rectangular, polygonal, or the like. For example,in the case where the electrode layer 4 has a circular cross section,i.e., in the case where the electrode is cylindrical, the diameter ofthe cross section is preferably in the range of 400 to 600 μm, and thedistance between the centers of the two electrode layers 4 and 4' ispreferably in the range of 0.5 to 2 mm. However, these values arevariable depending upon the applications thereof.

The electrode layer 4 can be formed by an electron beam vapor depositionmethod, a sputtering method or the like. For example, in the case wherethe electrode layer 4 is formed by the electron beam vapor depositionmethod, the ultimate background pressure is preferably 1×10⁻⁷ Torr orless, and the pressure during the deposition is preferably 5×10⁻⁷ Torror less.

The electrode layer 4 can be formed by simultaneously depositing thenitride forming metal and the hydrogen absorbing metal on thesemiconductor layer using the above-mentioned method, or the electrodelayer 4 can also be formed by depositing an intermetallic compoundcontaining the nitride forming metal and the hydrogen absorbing metal onthe semiconductor layer. Alternatively, the electrode layer 4 may beformed by sequentially depositing the nitride forming metal and thehydrogen absorbing metal in this order (or in an inverse order) on thesemiconductor layer, using the above-mentioned method. The electrodelayer 4 may also be formed by alternately depositing the nitride formingmetal and the hydrogen absorbing metal several times on thesemiconductor layer, using the above-mentioned method.

FIG. 2 shows an exemplary state where a metal hydride layer 7 and ametal nitride layer 8 are stacked in this order as the electrode layer 4on the semiconductor layer 3. Herein, GaN is used for the semiconductorlayer 3; Pd is used for the metal hydride layer 7; and Ti is used forthe metal nitride layer 8.

A chemical reaction proceeds at the interface between the electrodelayer 4 consisting of the Pd layer 7 and the Ti layer 8 and the GaNlayer 3, i.e., the interface between the Pd layer 7 and the GaN layer 3.This chemical reaction can be caused when the Pd layer 7 and the Tilayer 8 are formed, or when the annealing is performed after the Pdlayer 7 and the Ti layer 8 are formed. That is to say, a metal hydridePdH can be generated by the chemical reaction between Pd and hydrogenexisting in the GaN layer 3, and a metal nitride TiN can be generated bythe chemical reaction between Ti and nitrogen existing in the GaN layer3. PdH and TiN thus generated can exist in the interface between theelectrode layer 4 and the GaN layer 3 in any of the following states:(1) A layer composed of the mixture of PdH and TiN exists in theinterface; (2) At least one layer containing PdH and at least one layercontaining TiN exist in the interface; (3) PdH exists in at least aportion of the interface and TiN exists in at least a portion of theinterface; or (4) A layer composed of a compound of PdH and TiN existsin the interface. The last state (4) is likely to be caused in the casewhere Pd and Ti are simultaneously deposited on the semiconductor layer3.

In a preferred embodiment, after the electrode layer 4 is formed, anannealing is performed. The annealing can be performed by an electricfurnace annealing method, a rapid thermal annealing (RTA) method or thelike. The annealing temperature is preferably in the range of 100° to1000° C., and more preferably in the range of 300° to 500° C. Althoughthe annealing time is variable depending upon the annealing temperature,the time is preferably in the range of 5 to 30 minutes, and morepreferably in the range of 5 to 10 minutes. By performing the annealingunder these conditions, the reaction in the interface between thesemiconductor layer and the electrode layer is promoted and the ohmiccharacteristics of the electrode structure are further improved.

Next, referring to FIG. 3, the interface portion between thesemiconductor layer 3 and the electrode layer 4 will be brieflydescribed below. FIG. 3 is an enlarged cross-sectional view showing theinterface portion between the semiconductor layer 3 and the electrodelayer 4.

As shown in FIG. 3, a semiconductor layer 5 having a high carrierdensity (a p⁺ -Al_(x) Ga_(y) In_(1-x-y) N:Si (0≦x≦1, 0≦y≦1, x+y≦1)semiconductor layer: hereinafter, this layer will be called a "contactlayer") is formed in the interface portion between the semiconductorlayer 3 and the electrode layer 4 in the electrode structure thusobtained.

The carrier density of the contact layer 5 has become larger than thecarrier density of the semiconductor layer 3 before the electrode layer4 is formed. That is to say, by forming the contact layer 5, it ispossible to realize an ideal ohmic contact showing an extremely smallcontact resistance in the interface between the semiconductor layer 3and the electrode layer 4. Specifically, the carrier density of thecontact layer 5 is preferably in the range of 10¹⁸ to 10²⁰ cm⁻³, andmore preferably in the range of 10¹⁹ to 10²⁰ cm⁻³.

According to the present invention, it is possible to realize an idealohmic contact showing an extremely small contact resistance at theinterface between the semiconductor layer 3 and the electrode layer 4.This effect can be obtained based on the following mechanism. The metalhydride in the electrode layer 4 attracts hydrogen existing in thesemiconductor layer 3 to the interface between the semiconductor layer 3and the electrode layer 4. As a result, the hydrogen atoms which havebeen bonded with Mg are attracted to the metal hydride, so that fresh Mgatoms are reproduced. In other words, the Mg atoms are activated. Theactivated Mg atoms function as an effective acceptor impurity, therebyforming a contact layer 5 having a high carrier density at the interfacebetween the semiconductor layer 3 and the electrode layer 4. Theexistence of the contact layer 5 thus formed considerably reduces thewidth of the potential barrier at the interface between thesemiconductor layer 3 and the electrode layer 4, thereby abruptlyincreasing the tunnel current flowing through the interface. As aresult, the contact resistance between the semiconductor layer 3 and theelectrode layer 4 becomes extremely small, and therefore an ideal ohmiccontact is realized.

In addition, according to the present invention, the metal nitride inthe electrode layer effectively attracts the nitrogen atoms existing inthe semiconductor layer 3 to the interface between the semiconductorlayer 3 and the electrode layer 4. Accordingly, even if the nitrogenatoms dissociate from the semiconductor layer 3 while the semiconductorlayer 3 is being formed, so as to form the vacancies in thesemiconductor layer 3, especially in the vicinity of the surface of thesemiconductor layer 3, the nitrogen atoms attracted to the vicinity ofthe interface by the metal nitride compensate for the vacancies byfilling them. Consequently, crystals having an appropriatestoichiometric ratio can be formed at the interface between thesemiconductor layer 3 and the electrode layer 4. As a result, theportion in the vicinity of the surface of the semiconductor layer 3 canbe effectively turned into a p-type semiconductor.

Moreover, according to the present invention, the semiconductor layer 3is bonded with the electrode layer 4 at the interface therebetweenbecause of the reaction. Accordingly, an excellent adhesiveness isrealized at the interface between the semiconductor layer 3 and theelectrode layer 4, so that the electrode structure of the invention hasexcellent physical and chemical properties. Such an electrode structurecan be heat-treated at a high temperature, e.g., about 1000° C. Thus theannealing can be performed more effectively in order to promote thereaction at the interface and the activation of Mg, so that the ohmiccharacteristics are further improved.

As described above, according to the present invention, since the metalhydride and the metal nitride exist in the electrode layer, it ispossible to obtain an excellent electrode structure, which has excellentphysical and chemical properties, and an excellent ohmic contact for ap-type semiconductor device.

Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings. It isnoted that the present invention is not limited to the followingspecific examples.

Example 1

In the first example, an electrode structure as shown in FIG. 1 isfabricated in the following way.

First, a buffer layer 2 made of GaN (thickness: 50 nm) is formed on asapphire substrate 1 by a MOCVD method. Next, a semiconductor layer 3made of an Mg-doped p-type GaN (thickness: 3 μm) is formed on the bufferlayer 2 by a MOCVD method. The carrier density of the p-type GaNsemiconductor is 1×10¹⁸ cm⁻³. The electrode layers 4 and 4' are thenformed on the semiconductor layer 3 by sequentially depositing a metalhydride layer 7 made of Pd and a metal nitride layer 8 made of Ti, inthis order, on the semiconductor layer 3 inside a vacuum depositionapparatus by an electron beam method. The thicknesses of the metalhydride layer 7 and the metal nitride layer 8 are set to be 20 nm and 50nm, respectively. In forming the metal hydride layer 7 and the metalnitride layer 8, the ultimate vacuum degree is 1×10⁻⁷ Torr, and thevacuum degree at the deposition is 5×10⁻⁷ Torr. The electrode layers 4and 4' thus formed have a circular cross section having a diameter of500 μm. The distance between the centers of these two circularelectrodes is 1 mm.

FIG. 4 shows the resulting current-voltage characteristics between theelectrode layers 4 and 4' of this electrode structure.

Comparative Example 1

In this comparative example, an electrode structure as shown in FIG. 1is fabricated in the same way as in the first example, except that Ni isused instead of Pd and Ti as the electrode layers 4 and 4'. Thiselectrode structure corresponds to one of the electrode structuresexhibiting excellent ohmic characteristics disclosed in JapaneseLaid-Open Patent Publication No. 5-291621. FIG. 4 also shows theresulting current-voltage characteristics between the electrode layers 4and 4' of the electrode structure of this comparative example, alongwith the results of the above-described Example 1.

As is apparent from FIG. 4, the electrode structure of Example 1exhibits much more ideal ohmic characteristics as compared with those ofthe electrode structure of Comparative Example 1.

Example 2

The effect of the annealing process on the electrode structure isinspected in the following manner. The electrode structure obtained inExample 1 is annealed in an electric furnace. The annealing process isperformed three times at temperatures of 100° C., 300° C. and 500° C.,respectively. The annealing time is set to be 10 minutes in any of theseannealing processes. FIG. 5 shows the respective current-voltagecharacteristics of the electrode structure annealed at the threetemperatures and those of the electrode structure of Example 1 which isnot annealed.

As is apparent from FIG. 5, the annealing process improves the ohmiccharacteristics. The result shown in FIG. 5 also indicates that thehigher the annealing temperature is, the more ideal the ohmiccharacteristics becomes, and that the ohmic characteristics areremarkably improved by performing the annealing process at 300° C. orhigher. The results shown in FIG. 5 also indicate that the electrodestructure of the invention has excellent physical and chemicalproperties. More specifically, the crystal structure in the vicinity ofthe interfaces between the semiconductor layer 3 and the electrodelayers 4 and 4' in this electrode structure can be annealed at 500° C.

Example 3

In the third example, an electrode structure as shown in FIG. 1 isfabricated in the same way as in the first example, except that Al₀.3Ga₀.7 N having a carrier density of 5×10¹⁷ cm⁻³ is used instead of GaNas the semiconductor layer 3, and that an intermetallic compoundcontaining Hf and Nb is used instead of Pd and Ti as the electrodelayers 4 and 4'. Accordingly, the electrode layers 4 and 4' of thiselectrode structure includes a metal hydride layer made of HfH₂ and ametal nitride layer made of NbN. FIG. 6 shows the resultingcurrent-voltage characteristics between the electrode layers 4 and 4' ofthe electrode structure of this example.

Example 4

In the fourth example, an electrode structure as shown in FIG. 1 isfabricated in the same way as in the third example, except that In₀.2Ga₀.8 N having a carrier density of 5×10¹⁷ cm⁻³ is used instead of Al₀.3Ga₀.7 N as the semiconductor layer 3. FIG. 6 also shows the resultingcurrent-voltage characteristics between the electrode layers 4 and 4' ofthe electrode structure of this example, along with the results ofExample 3.

As is apparent from FIG. 6, both the electrode structures obtained inExamples 3 and 4 exhibit ideal ohmic characteristics. In particular, theelectrode structure obtained in Example 4 exhibits excellent ohmiccharacteristics.

Example 5

In the fifth example, an electrode structure as shown in FIG. 1 isfabricated in the same way as in the first example, except that variouscombinations of the nitride forming metals and the hydrogen absorbingmetals shown in the following Table 1 are used as the electrode layers 4and 4'. The resulting electrode structure is then annealed at 500° C.for ten minutes. The current-voltage characteristics between theelectrode layers 4 and 4' of the resulting electrode structure areinspected so as to obtain the resistance values shown in Table 1.

                  TABLE 1                                                         ______________________________________                                             Sc     Ti      V    Cr    Zr   Nb    La   Ta                             B    (-61)  (-74)   (-35)                                                                              (-24) (-87)                                                                              (-51) (-65)                                                                              (-54)                          ______________________________________                                        Y    29.3   20.2    62.5 70.8  15.8 40.5  28.7 36.7                           Pd   15.0   13.9    35.6 50.3  10.0 17.2  14.5 16.3                           Ce   29.7   28.1    74.0 98.7  20.3 34.0  30.8 35.2                           Pr   43.8   35.6    55.3 60.2  30.5 38.3  44.3 37.2                           Nd   49.7   38.6    60.3 70.6  31.8 40.7  48.3 39.3                           Sm   15.1   14.2    36.0 48.3  11.0 18.3  15.9 17.2                           Eu   22.6   21.3    65.3 72.7  20.7 35.3  30.5 35.2                           Gd   20.4   15.8    58.7 60.3  15.6 30.3  24.6 25.2                           Tb   13.7   14.0    38.3 48.1  11.3 18.1  15.7 17.3                           Dy   47.2   45.3    87.1 87.3  24.3 48.3  47.1 49.2                           Ho   37.6   18.8    52.7 55.3  17.6 41.7  40.3 42.6                           Er   35.1   18.7    53.5 56.2  18.6 36.8  32.3 34.4                           Tm   27.6   20.3    66.3 65.3  21.7 30.7  24.3 30.4                           Yb   29.9   27.8    39.3 40.8  25.6 34.6  29.3 31.6                           Hf   50.2   44.3    72.9 80.3  39.3 53.7  48.7 50.7                           ______________________________________                                         (unit: Ω)                                                               A: Nitride forming metal                                                      B: Hydrogen absorbing metal                                                   Note: The numerals in the parentheses shown under the respective nitride      forming metals denote the variation amount (kcal/mol) of the free energy      when the metals are nitrified.                                           

As is apparent from Table 1, the electrode structure using the nitrideforming metal and the hydrogen absorbing metal shows low resistancevalues, i.e., excellent ohmic characteristics.

As described above, according to the present invention, it is possibleto obtain an electrode structure having an ideal ohmic contact showingan extremely small contact resistance between the semiconductor layerand the electrode layer. In addition, crystals having an appropriatestoichiometric ratio are formed in the interface between thesemiconductor layer and the electrode layer, so that an electrodestructure having excellent physical and chemical properties can beobtained.

By using such an electrode structure, a semiconductor device operatingat a lower operational voltage and exhibiting more ideal resistivity,e.g., a blue-light-emitting diode, can be obtained as compared with thecase of using a conventional electrode structure.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An electrode structure comprising a p-type Al_(x) Ga_(y) In_(1-x-y) N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer and an electrode layer formed on the semiconductor layer,wherein the electrode layer contains a mixture of a metal nitride containing a first metal and a metal hydride containing a second metal, wherein the first metal and the second metal are different.
 2. An electrode structure according to claim 1, wherein the metal nitride containing the first metal is selected from a group consisting of ScN, TIN, VN, CrN, ZrN, NbN, LaN and TaN.
 3. An electrode structure according to claim 1, wherein the metal hydride containing the second metal is selected form a group consisting of YH₂, CeH₂, PrH₂, NDH₂, SmH₂, EuH₂, YbH₂, HfH₂, PdH, TmH, ErH, HoH, DyH, TbH and GdH. 